Fight the good fight and stay focused on just playing squash. The longer a game goes, the chances of either player getting frustrated, tired, or starting to feel hopeless, increases exponentially. As the body tires and the focus on playing squash diminishes, players at all levels may let up and the match soon ends in their opponent's favor. They run out of options, and they crumple mentally.
People get tired, especially if you have been running them along the diagonals of the court. This leads to looser shots aka opportunities for you to be more aggressive. Trust in your gameplan, it's about to work. A confident game plan, allows you to focus on just hitting the ball, the execution, so stick with it and remove the indecision. Only when you are certain, should you change a losing game plan and when you do, change it up dramatically.
Give your opponent lots of opportunities to give up or get distracted. It's amazing how a lapse in focus can lead to a momentum swing in a squash match. It's very common for players who just won one game to let up significantly at the start of the next. By extending points, slowing the pace or taking time between points, you allow your opponent to think about how nice winning will be instead of focusing on the match itself. If your down, allow the tide to turn by retaining your focus on playing squash. Think squash thoughts, not thoughts about the score, or whether you're embarrassed, or how your team-mates are doing. Keep your focus.
Learn to spot weaknesses and how to exploit them. Really look at how unbalanced your opponent's shots are, or if they are weaker on one side, it they are slow moving to the front of the court, if they don't like receiving a hard serve, or a very soft lob serve. Vary up your shots until you find a weakness to exploit, a good time to do this is warmup. Conversely, don't let them hit the shots they like to hit. Minimize opportunities for them to capitalize on their strengths, and allow them to hit lots of their worst strokes.
Concentrate on hitting better shots as they may start to work better. Sometimes you are just off, shots clip the the top of the tin, boasts sit up too high, or you're length comes off the back wall. Focusing on improving your shots will help shift the momentum and you may find the proper length and control on your shots before the match ends.
Draw out long points and your opponent may go brain-dead and start hitting stupid shots. An exhuasted opponent will start hitting without thinking. When your opponent is too exhausted to think, you're in a good competitive position.
Changing ball condidtions may work in your favour. Long, hard rallies heats up the ball making it hard to hit controlled drop shots. If your opponent has been hurting you with drop shots while the ball is still cold, take a step upcourt. If the ball starts to get hot, be confident that the hot ball drop shot will be easier to get than the cold ball drop shot was.
Look to get a second wind and be fitter in a long match. Fitness is key in squash. Condition yourself in the weeks and months before your big matches, then rest right before your match. Give yourself a full tank to help you win the endurance contest in a five-game match. (Create the opportunities to catch your breath by hitting high slow lobs. Conversely, take away your opponent's time by volleying whenever possible.
Start to analyze your opponents mental state, they might be about to give up, or be mentally fragile. There's no way of knowing exactly what's going on inside your opponent's head. Trust your gut, if you sense their frustration, or exasperation, continue to apply pressure, keep getting to every ball, and work as hard as possible on every shot on every point, you never know when they are going to crack, it might happen at any time.
If you've got a full tank, use it...why leave any energy unspent? There is no point in saving yourself for another match, especially if lose. Short of injuring yourself, you should try as hard as possible to get every ball and win every point. Put out your very best effort every time you play a match, and remember that the match isn't over until someone wins the very last point.